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Friday, August 15, 2014

The mysterious case of the prickly palo verde

My infatuation with the three palo verde trees in front of our house continues unabated. I love running my hands through the soft, delicate foliage.

I did just a few days ago with the ‘Desert Museum’ palo verde (Parkinsonia ‘Desert Museum’) in the succulent bed between our house and our neighbor’s when I felt a prick on my skin. Not just one, actually, several. Hmmm, I thought, the ‘Desert Museum’ hybrid doesn’t have thorns. What could it be?

I took a closer look at the leaves.

Normal enough.

But then I saw what is shown in the next photo. Take a look at the branchlets hanging down from the horizontal branch you see at the top of the photo. And then take a look at the branch that comes in from the right in the lower ⅓ of the photo. The leaves are completely different.

The leaves from the horizontal branch on top are significantly smaller.

In fact, they’re positively tiny!

It didn’t take long to realize that there was one branch—what I thought of as a second trunk—coming from the rootstock below the union. (‘Desert Museum’ palo verde are almost always grafted.)

Out came the saw, and a minute later the offending “water shoot” had been removed.

Looking at the leaves—tiny, sparse and oblong—I think the rootstock is from a Mexican palo verde (Parkinsonia aculeata).

And here are the thorns that had pricked me. Fittingly enough, Parkinsonia aculeata is also called “Jerusalem thorn.”

There are four species of palo verde: Mexican palo verde (Parkinsonia aculeata), blue palo verde (Parkisonia florida), foothill palo verde (Parkinsonia microphylla), palo brea (Parkinsonia praecox), and the lesser known Texas palo verde (Parkinsonia texana). This article gives a good overview of the various varieties. (To clear up some taxonomical confusion, palo verdes used to be in the genus Cercidium, which has now been subsumed into the larger genus Parkinsonia. However, most nurseries still sell palo verdes as Cercidium.)

Each of these species has its unique advantages and disadvantages so choosing the perfect one was going to be difficult. Then I stumbled upon what would become my holy grail: a naturally occurring palo verde hybrid called ‘Desert Museum’. This cross between the Mexican palo verde (Parkinsonia aculeata), the blue palo verde (Parkinsonia florida), and the foothill palo verde (Parkinsonia microphylla) was discovered over 30 years ago by Mark Dimmitt, the directory of natural history at the Arizona-Sonora Desert Museum in Tucson, Arizona. (Read this article to find out more about the history of the ‘Desert Museum’ palo verde.)

The ‘Desert Museum’ hybrid grows to a very manageable 20-25 ft. and combines the best qualities of each of its parents:

It is cold hardy to 15°F.

It is a fast grower given enough heat (5-6 ft. per year in the ground).

It is very drought-tolerant although deep watering in the summer (every two weeks or so) helps it grow faster.

It has larger and brighter flowers.

It flowers for a longer period of time (four or five weeks in the spring) and often blooms again in the summer.

It is thornless.

Its trunk is a deeper green. (As much as ¾ of the tree’s photosynthesis occurs in the chlorophyll-bearing bark.)

It can handle clay soils as long as its roots aren’t waterlogged for extended periods of time.

That's a great size of tree for a smaller garden, and 20-25' still provides shade. I've seen a few DMs planted around here, though they are better with more heat, as at the Huntington, which is 10+ degrees warmer than here on a summer day.

It certainly looks like our three palo verdes are getting all the heat they want/need. I've turned off the drip emitters I'd installed for them when they were smaller. Now they can look for water where they can find it--not difficult since they are planted in beds with drip irrigation.